1. Field of the Invention:
This invention relates to an apparatus for magnetically detecting the position or speed of a moving body by using a magnetoresistive element, and more particularly to an apparatus for obtaining a sine wave output signal for detecting the position or speed of a moving body.
This invention can be applied to a rotationally moving body, a linearly moving body and a sinuously moving body.
Namely, this invention is directed to an apparatus for detecting the angle of rotation of various types of rotary shafts, to say nothing of the angle of rotation and speed of, for example, a motor. The apparatuses, to which the present invention can be applied to detect the position or speed of a linearly moving body, include a position detector in a printing head of a printer and a head of a magnetic disc drive. Generally speaking, the present invention can be utilized widely for a speed controlling apparatus or various types of positioning apparatus.
2. Description of the Related Art:
An apparatus for detecting the position and angle of a moving body by using a magnetoresistive element (which will hereinafter be referred to as a MR element) which is adapted to sense magnetism and cause variation in the internal electric resistance thereof is well known. The invention disclosed in U.S. Pat. No. 4,319,188 is also an example of an apparatus for detecting the angle of a moving body by using a MR element. The internal electric resistance of a MR element varies depending upon the intensity of a magnetic field working thereon. Therefore, when the air gap between a MR element and a magnetic medium having a plurality of magnetic poles and moving as it is opposed to the MR element varies, an output signal of the MR element also varies based on the variation of the internal electric resistance. For example, the true circularity of a rotary magnetic medium cannot be obtained even if the magnetic medium is manufactured with the highest possible accuracy. The magnetic medium also has unbalanced mass and causes the rotary system to have play and eccentricity. Consequently, when the magnetic medium is rotated, the air gap varies in each turn thereof to cause an output signal from the MR element to be distorted.
In order to obtain a high-level output signal from a MR element, the air gap mentioned above must be reduced. However, if the air gap is reduced excessively, the MR element is magnetically saturated to cause its output waveform to be largely distorted, so that the output waveform becomes dissimilar to a sine waveform. Namely, in general, a magnetic sensor utilizing a MR element has the following characteristics. When the originally narrow air gap varies slightly the output amplitude does not vary but the output waveform is distorted due to the magnetic saturation. Conversely, when the air gap is originally wide the waveform of an output from the sensor is not distorted but the output amplitude varies when the air gap varies.
Thus, when an attempt in obtaining a high-level output from a known angle detector utilizing the MR element is made, an output, the waveform of which is distorted, is obtained and an output of a sine waveform is not. When an attempt in obtaining an output signal of a sine waveform having no distortion is made, a high-level output signal cannot be obtained.